Measurements Of Photon Flux Density Research Articles

Spatial heterogeneity and diurnal course of photon flux density on paddy field water surface under rice plant canopy

Detailed measurements of photon flux density (PFD) on the paddy field water surface were conducted under sunny sky conditions using a photosensor array of 100 small photodiodes under rice canopies at the heading and ripening stages. PFD on paddy water surface was not uniform and spatially variable even within a short distance of a few centimeters, showing that the irregular penetration of direct sunlight significantly affects the light environment under the canopies. Although occasional high peaks of PFD were observed, most of the water surface was exposed to a PFD of less than 200 µmol m−2 s−1, even around midday. Mean PFD were high at midday and low in early morning or late evening. Mean ratios of the PFD under the canopy to that outside the canopy (RPFD) were also high at midday and low in early morning or late evening, which can be explained by Poisson's model of light penetration. Maximum mean PFD and mean RPFD were often observed not around noon, but when the azimuth angle of the sun coincided with the row direction. Moreover, under rice canopies at the heading stage, rapid increases and decreases of mean RPFD were observed immediately before and after the peak time. These results indicate that significant high photosynthetic carbon fixation by the paddy field weeds can take place only during the few hours when the azimuth angle of the sun coincides with the row direction.

Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models

The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance (PI) measurements and continuous measurements of underwater photon-flux density (PFD). The validity of applying the H sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis (P max) and daily hours of saturating irradiance (H sat) was assessed for T. testudinum by comparison with integrated production estimates derived through numerical integration. Gross integrated production values were combined with dark-respiration measurements of photosynthetic (PS) and non-photosynthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H sat model significantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross integrated production ranged between 11.5 mg C g−1 leaf dry wt d−1 in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g−1 leaf dry wt d−1 in July. Modeled net carbon gain was highest in July at 454 mg C m−2 d−1 (1.4 g dry wt m−2 d−1), sufficient to account for measured rates of leaf production in the study area and representative of T. testudinum populations of low productivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration (P net:R d) ratios, sporadic low-light periods, the small fraction of PS tissue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as reflected in whole-plant P net:R d ratios.

Deep occurrence of the giant kelp Macrocystis laevis in the Southern Ocean

During a combined hydroacoustic-trawling survey at the Prince Edward Arch~pelago, a kelp bed of Macrocystis laevis growing at a maximum depth of 68 m was repeatedly observed in the open shelf area This is the deepest population ever described for any species of giant kelp. Nutrient availability and temperature in the area were found to have no significant impact on kelp production and distribution. Likewise, measurements of photon flux density showed that neither plant growth nor sporophyte formation was likely to have been light-inhibited during the period of the observations. However, since irradiance levels in the deep kelp bed were close to the lower threshold for growth, the absence of plants below 68 m depth was probably determined by light availability. Around Marion Island, where M. laevis is found only within the 20 m isobath, the offshore limit of the kelp beds is controlled by availability of suitable rock substrata. The inshore limit appears to be set by the maximum height of storm-induced waves. Four species of Macrocystis are generally recognized: M, integrlfolia, M, angustifolja, M. pyrifera and the recently described M. laevis (Womersley 1954, Neushul 1971, Hay 1986, Brostoff 1988). Macrocystis kelps are found in cold temperate oceans with surface temperatures varying from 0 to 20 OC, and their main distribution is circumpolar in the Southern Hemisphere, between 40 and 60 S (Table 1; Cribb 1954, Michanek 1983). They are also present in the major coastal upwelling areas. On the west coast of South Africa they occur between 3324' and 348'S and on the Pacific coast of South America extend almost to the equator, reappearing in North America along Baja California and reaching southern Alaska to the north (Table 1). Only 2 species of Macrocystis are represented in the Southern Ocean: M. pyrifera and M. laevis. While M. pyrifera is widely distributed from subantarctic South America to New Zealand, and occurs at most islands in the Atlantic and Indian sectors of the Southern Ocean, M. laevis is endemic to the Prince Edward Archipelago

PHOTOSYNTHESIS/PHOTON FLUX DENSITY RELATIONSHIPS AMONG COMPONENTS OF CORAL REEF ALGAL TURFS1

ABSTRACTPhotosynthesis/photon flux density (PFD) relationships were compared among some of the major components of sparse algal turfs (green endoliths, red crusts, red filaments, Crouania, Sphacelaria)/ from Tague Bay forereef St. Croix, U.S. Virgin Islands. Algal turfs grazed by the sea urchin Diadema antillarum were 2–10 times more productive per unit chl a than when not grazed by sea urchins. The maximum rate of net photosynthesis (Pnetmax) and saturation PFDs, but not the slope of the light‐limited portion of the curve (α), differed significantly among the algal turf components examined. The hypothesis that increased biomass‐specific primary productivity results from shifts in algal community structure was not supported because the maximum difference in photosynthesis between algal components was only a factor of two, and the less productive components were relatively more abundant under grazing. In the understory, green endoliths exhibited higher α and lower Pnetmax, suggesting shade adaptation. Photon flux density measurements taken with a fiber optic microprobe within the algal turfs indicated that photosynthesis of basal portions of algal filaments and red crusts are light‐limited in ungrazed algal turfs. As self‐shading changes with grazer‐mediated canopy removal, algal sublayers will contribute differentially to whole turf primary productivity.

Nitrogen use efficiency in instantaneous and daily photosynthesis of leaves in the canopy of a Solidago altissima stand

Photosynthetic capacity was measured on detached leaves sampled in a canopy of Solidago altissima L. Non‐rectangular hyperbola fitted the light response curve of photosynthesis and significant correlations were observed between leaf nitrogen per unit area and four parameters which characterize the light‐response curve. Using regressions of the parameters on leaf nitrogen, a model of leaf photosynthesis was constructed which gave the relationships between leaf nitrogen, photon flux density (PFD) and photosynthesis. Curvilinear relations were obtained between leaf nitrogen and photosynthetic rate on both an instantaneous and a daily basis. Nitrogen use efficiency (NUE, photosynthesis per unit leaf nitrogen) was calculated against leaf nitrogen under varying PFDs. The optimum nitrogen content per unit leaf area that maximizes NUE shifted to higher values with increasing PFD. Field measurements of PFD showed high positive correlations between the distribution of leaf nitrogen in the canopy and relative PFD. The predicted optimum leaf nitrogen content for each level in the canopy, to achieve maximized NUE during a clear day, was close to the actual nitrogen distribution as found through sampling.